In petroleum and gas recovery, fractures are commonly created in reservoir rock surrounding a well in order to stimulate production rates. In order for the high pressure fracturing fluids to reach the rock, the casing of a well is perforated with explosive charges placed at the desired depths. The charges are generally shaped, designed and oriented to concentrate their force on single points, and accordingly tend to make circular holes or perforations in the casing and cement sheath. The holes are frequently small, i.e. typically one-fourth to one-half inch in diameter, which tends to cause large pressure drops when the fracturing fluid is forced into the well, reducing the effect of the hydraulic fracturing effort. Larger holes are not considered desirable because they tend to weaken the casing wall unacceptably. The pressure drop limits the flow rate and amount of fluid which can be forced into the formation during fracturing. Moreover, a relatively large number of perforations may frequently be used because of the aforesaid limitations, and the multiplicity of perforations results in relatively numerous small fracture initiation points having various orientations. This results in poor connection between the initiation points and the well bore, a condition sometimes called "near well bore tortuosity", and dissipates the pressure of the fracturing fluid in areas immediately around the well bore rather than extending relatively fewer, larger fractures. A single long fracture, rather than numerous small, tortuous fractures, is desirable.
While linear apertures have been made in well bores by abrasive fluids (see U.S. Pat. No. 5,335,724), we are not aware of the previous use of linear shaped charges, sometimes herein called linear charges, in the preparation of hydrocarbon recovery wells for fracturing. Linear charges themselves are not new. See, for example, Alford's U.S. Pat. No. 5,036,771, which describes a kit for assembling a linear charge to be used for demolition. Such charges are used in building implosions, underwater demolition, and elsewhere where it is desired to direct the energy of the charge to make a linear cut rather than a circular one or undirected explosion. The linear orientation of the charges described by Hayes in U.S. Pat. No. 4,881,445 is not the same--the patentee there suggests simply a string of spaced circular charges. Similar effects are obtained by the use of so-called bi-wire strip charges and hollow steel charge carriers commonly used in the art, both of which are designed for the emplacement of a series of spaced shaped charges for perforating the well casing with circular holes. These are not linear charges but merely orderly rows of circular charges, resulting in rows of spaced circular perforations. The technique known as overbalance perforating, using circular apertures--that is, conventional perforating charges--is described by Dees, Handren and Jupp in U.S. Pat. No. 5,131,472. In overbalance perforating, perforation is performed under high internal pressures and fracturing is begun immediately after perforation. When used herein, the term overbalance perforating, is intended to include the steps of pressurizing before perforating and fracturing immediately after perforating.
The hydrocarbon recovery art is in need of a technique and means for overcoming the disadvantages of conventional perforation of well casings.